1. Field of the Invention
The present invention relates to methods of conferring resistance to parasites, such as viruses, bacteria, and higher parasites, to hosts of the parasite. More particularly, the present invention relates to viral resistance obtained by genetic engineering of the host organism to contain a portion of a replicase enzyme from an RNA virus.
2. Description of the Background
A potentially important application of genetic engineering technology is in the area of producing resistance to parasites. The proposals in the prior art that have been systematic and broadly applicable have centered on finding a gene conferring resistance within a strain of the host species or within a related species and transforming the gene into the genome of a susceptible host. This approach may prove effective but has several distinct disadvantages. Resistant forms of the host may not exist or may be very difficult to find for each new race of parasite which arises. Such resistance may be polygenic, making the cloning and transfer of the resistance genes difficult. Where resistance is encoded by a gene, there are commonly already strains of the parasite that have evolved virulence genes for overcoming such host-derived resistances in a gene-for-gene fashion (Flor 1971). Finally, the problem of identifying and isolating the resistance gene from within the large genome of the host will generally remain very difficult. An alternative strategy that addresses these problems is therefore needed.
There have also been proposals for and some work on using genes from organisms unrelated to either host or parasite, which serendipitously have gene products detrimental to a specific parasite. The gene coding for the endotoxin of Baccillus thuringiensis (which is toxic to lepidopterous insects) would be an example of this (Held et al., 1982). While this type of approach may prove useful in some specific cases, it clearly represents an opportunistic approach to the problem, as opposed to a systematic methodology that can be applied very broadly.
There already exist some examples of genes, gene derivatives, or gene products of a parasite that can produce a negative interaction with itself or a related genotype. Studies into the susceptibility of plants to infection by viruses have demonstrated that closely related plant viruses or different strains of the same virus will cross-protect a host organism (Hamilton, 1980). In other words, a plant infected by a first virus is often not subject to infection by a second strain of that virus or by a related virus. A similar phenomenon has been observed in animal viruses and has been termed intrinsic interference (Marcus and Carrier, 1967). From the point of view of parasite resistance of the type discussed herein, the key proteins involved in the intrinsic interference phenomenon are the viral replicase proteins (Marcus and Zuckerbraun, 1970). These same authors proposed that the replicase proteins of the primary infecting virus prevent the replication of the second virus by binding to its replicase attachment sites (Marcus and Zuckerbraun, 1969). A similar proposal has been put forth to explain cross-protection in plants (Gibbs, 1969). In a similar manner, experimenters working with an E. coli infected with bacteriaphage 434 have found that infected bacteria are immune to other phages (Lauer et al, 1981; Flashman, 1978; Roberts et al, 1979). Other workers have noticed that endogenous as well as experimentally introduced complementary oligonucleotides can interact with mRNA in a potentially detrimental manner. Simons and coworkers (1983) have suggested that hybridization of a small anti-sense transcript to E. coli Tn10 mRNA contributes to the regulation of transposition of that element. Stephenson and Zamecnik (1978) and Zamecnik and Stephenson (1978) have shown that synthetic oligodeoxynucleotides, complementary to Rous sarcoma virus terminal repeats, diminish normal viral infection and can inhibit viral RNA translation in vitro. However, these discoveries were not applied to the production of host resistance to a parasite.
Despite this fragmentary knowledge in the prior art, there still remains a need for a fully developed technique for producing resistance to parasites that is not based on the traditional methods of using a resistance gene from an immune strain of a host.